This invention relates to a scaling method for an automatic welding machine, particularly a scaling method for an automatic welding machine that makes scaling possible on a three-dimensional, gently curved surface.
An automatic welding machine performs welding by impressing a voltage across a wire and a workpiece to produce an arc at the tip of the wire, and moving the wire tip along a welding path while the wire is successively paid out in small increments. FIG. 1 is a simplified view of such a welding machine. In the Figure, WR denotes the wire, which is paid out in small increments in the direction of the arrow by feed rollers FR, so that its tip protrudes from the end of a torch TC via guide member GB, with the amount by which the wire is fed being controlled in such a manner that the tip comes to occupy a position spaced a predetermined distance from the surface of the workpiece WK. PS represents a welding power supply for generating an intermittent high voltage having a predetermined period. The plus side of the high-voltage is applied to the wire WR through the guide member GB, and the minus side is applied to the workpiece WK. In the welding machine, carbon dioxide gas (CO.sub.2) from a gas supply unit (not shown) is supplied through the interior of the torch TC in the direction of the arrows so as to impinge upon the workpiece and prevent the oxidation thereof.
When a high voltage is generated intermittently by the welding power supply PS while the carbon dioxide gas (CO.sub.2) is fed from the gas supply unit and the wire WR is paid out in small increments, an arc is produced at the tip of the wire WR, and both the wire and the workpiece are melted to weld the fused portions of the workpiece together. Recently, welding robots in which such a welding operation is performed by a robot, have come into use. Specifically, a welding robot performs welding by grasping the torch of a welding machine and causing the robot to move the torch (the tip of the wire) along a welding path. In order for such a welding robot to perform welding at predetermined locations on the workpiece, it is necessary for the robot to be taught the positions to which the torch is to be moved, the torch traveling speed, and the like. Also, there are various welding modes. When welding is merely performed once in linear fashion, teaching the robot is simple but the weld formed by such welding is weak in strength. Accordingly, so-called multi-layer welding, in which welding is repeatedly performed in overlapping fashion, is frequently used. In a case where such multi-layer welding is performed, the robot uses primary taught position data, which serves as a basis for welding in one plane, to derive all subsequent taught position data. This will be described in further detail with reference to FIG. 2. FIG. 2 is a sectional view of welding starting points for a multi-layer welded portion in a multi-layer welding operation. In the Figure, W1, W2 . . . W6 denote welding starting points of respective layers. The welding starting point W1 of the first layer is the vertex of the welded portion (groove). The operator teaches position data indicative of the welding starting point W1 as primary taught data. The point W1 serves as a foundation on the basis of which taught position data from this point onward are obtained. Scaling refers to the process of obtaining subsequent taught positions on the basis of primary taught position data.
Conventionally, scaling is premised on the fact that a group of primary taught points, which serve as the basis for scaling in order to perform automatic welding, are taught on a single plane. In other words, according to the conventional scaling method, an offset width becomes discontinuous at a line of intersection between planes when the points are not taught in a single plane. As a result, scaling cannot be carried out. The reason for this will now be described with reference to FIG. 3 and FIGS. 4(a), (b).
FIG. 3 is a diagram for describing a problem encountered when it is attempted to apply scaling to a group of taught points P1-P6 by offsetting these points outwardly by an offset width .sub..DELTA. W over three surfaces of a rectangular parallelepiped. FIGS. 4(a), (b) are partially enlarged views taken at the point P2 in FIG. 3.
Let us now describe the problems of the prior art based on these drawings. When it is attempted to perform scaling of offset width .sub..DELTA. W from two surfaces A, B at a point P2 lying at the intersection of the surfaces of the rectangular parallelepiped, a scaling width .sub..DELTA. x.sub.1 at the edge of surface A is obtained as follows: EQU .sub..DELTA. W/.sub..DELTA. x.sub.1 =sin .theta..sub.1 .thrfore..sub..DELTA. x1=.sub..DELTA. W/sin .theta..sub.1
where .theta..sub.1 represents the angle between a line along which welding is performed and the edge formed by the surfaces A, B, and .sub..DELTA. W represents the offset width.
Scaling width .sub..DELTA. x.sub.2 at the edge of surface B is obtained as follows: EQU .sub..DELTA. W/.sub..DELTA. x.sub.2 =sin .theta..sub.2 .thrfore..sub..DELTA. x2=.sub..DELTA. W/sin .theta..sub.2
where .theta..sub.2 represents the angle between a line along which welding is performed and the edge formed by the surfaces A, B, and .sub..DELTA. W represents the offset width. Accordingly, as long as the angles .theta..sub.1, .theta..sub.2 are unequal, .sub..DELTA. x.sub.1 =.sub..DELTA. x.sub.2 will not hold and the taught data set by scaling will be discontinuous at the edge portion of the rectangular parallelepiped. This makes scaling impossible.
This generally holds true even for three-dimensional curved surfaces, and the state of the art is such that scaling cannot be performed on three-dimensional curved surfaces.
However, cases are increasing in which welding is being applied to workpieces having three-dimensional curved surfaces, such as the gentle curved surfaces of automobile bonnets and the like. The fact that scaling cannot be applied to such workpieces limits scaling solely to perfect planes, thus diminishing the utility value of welding robots.